This invention relates to a pressure monitoring method and apparatus of an injection molding machine and more particularly to a method and apparatus for monitoring an abnormal rise of an injection pressure during an injection process of an injection molding machine thereby to protect a molding machine and molds against the abnormal or undesirable pressure rise.
An injection pressure at the injection operation of the injection molding machine is generally determined in accordance with a flow resistance of a material resin based on a shape of a mold to be used and an apparent viscosity of the material resin, so that an injection pressure is initially relatively low and gradually rises as the material resin fills the mold cavities, and finally, a profile representing substantially constant injection pressure is continuously detected during one injection operation cycle of the injection molding machine.
In a case where a plurality of mold products or a relatively large one mold product are to be obtained by an injection molding machine provided with a plurality of injection gates, a few of which are clogged with the material resin by a certain reason during the injection operation, flow speed of the material resin passing the other gates is obliged to increase and the injection pressure also abnormally rises, which may result in non-conformity of mold products and, sometime, in breaking of the molds or molding machine.
In order to obviate these defects or disadvantages in the injection molding operation, there is known a monitoring device which generates a signal for informing of the fact that an injection pressure during the operation abnormally exceeds a predetermined injection pressure. However, as described before, since the injection pressure, in fact, gradually increases as the material resin fills the molds during the injection process, it is difficult to accurately predetermine an injection pressure as a constant set pressure which is deemed as a standard for generating an alarm signal for informing of an abnormal pressure condition at a time when the injection pressure exceeds this set pressure. Moreover, there will occur a case where an abnormal pressure condition is not informed of in spite of generation of an abnormal pressure rise during the injection operation or a case where a signal indicating an abnormal pressure rise is generated in spite of a normal injection pressure condition. The reason why such undesirable phenomena occurred will be understood from the following description with reference to FIGS. 1 and 2.
Referring to FIG. 1, which shows a graph having an abscissa axis of time and an ordinate axis of injection pressure and in which four curves 1 through 4 representing injection pressure transitions with time are described. In these pressure curves, only the curve 1 shows a normal pressure transition with time and the curve 2 shows a curve inclusive of allowable upper limit of the curve 1. In use of a conventional monitoring apparatus, when a set pressure is now predetermined as constant value H.sub.1, an abnormal injection pressure condition such as shown by the curve 3 which exceeds the set pressure H.sub.1 can be detected, but the injection pressure condition such as shown by the curve 4 cannot be detected as an abnormal pressure condition because the curve 4 does not exceed the set pressure H.sub.1 in spite of the fact that the curve 4 shows an abnormal pressure condition. On the other hand, a signal informing of an abnormal pressure condition may be generated at a time when an injection pressure rises as shown by a portion M on the curve 2 in spite of the fact that the portion M is within the allowable pressure range. These adverse phenomena in a conventional monitoring apparatus will reside in that a set pressure for detecting an abnormal pressure rise is decided to be always constant in spite of the fact that an injection pressure in the abnormal condition will vary with time.
Referring to FIG. 2 which shows a graph having an abscissa axis of time and an ordinate axis of injection pressure and shows a curve 11 representing an injection pressure transition with time during a certain normal injection operation. In a case where it is now required to monitor the injection pressure at the section A to confirm that the pressure curve 11 in the section B within the section A will not exceed the highest injection pressure portion N in the section A by predetermining a certain set pressure H.sub.2 which is somewhat lower than the highest pressure portion N, in other words, where it is required to monitor the injection pressure so that although the injection pressure curve 11 in the section (A - B) is allowed to exceed the set pressure H.sub.2, the injection pressure curve 11 in the section B is not allowed to exceed the set pressure H.sub.2, an alarm signal will be generated in use of a conventional monitoring apparatus because the injection pressure portion N in the section (A - B) exceeds the set pressure H.sub.2. This is because the set pressure H.sub.2 is decided to be constant in spite of the fact that the injection pressure varies with time.